In a variety of manufacturing and production settings, there is a need to control alignment between various layers or within particular layers of a given sample. For example, in the context of semiconductor processing, semiconductor-based devices may be produced by fabricating a series of layers on a substrate, some or all of the layers including various structures. The relative position of these structures both within a single layer and with respect to structures in other layers is critical to the performance of the devices. The misalignment between various structures is known as overlay error.
Conventional overlay metrology systems, such as imaging or scatterometry based systems, are typically based on bright field illumination microscopy in which a dedicated metrology target, containing spatial information from at least two separate process steps is imaged onto a two dimensional sensor array. FIG. 1 illustrates a conventional overlay metrology system 100. The system 100 may include an illumination source 102 (e.g., broadband or narrowband source), a set of illumination optics 108, a beam splitter 104 configured to direct a light beam 112 to an objective 114, which in turn focuses the light onto one or more targets 117 of the wafer 116 disposed on a sample stage 118. The light is then scattered from a metrology target 117 of the wafer 116 and is transmitted along the imaging path 110 onto an imaging plane of the detector 106. Some metrology systems consist of a two beam (e.g., illumination path and reference path) interferometric configuration. Conventional two-beam metrology systems include a set of reference optics 120, which include, but are not limited to, a reference mirror, a reference objective, and a shutter configured to selectively block the reference path 122. In a general sense, a two-beam interference optical system may be configured as a Linnik interferometer. Linnik interferometry is described generally in U.S. Pat. No. 4,818,110, issued on Apr. 4, 1989, and U.S. Pat. No. 6,172,349, issued on Jan. 9, 2001, which are incorporated herein by reference.
The measurement of overlay error between successive patterned layers on a wafer is one of the most critical process control techniques used in the manufacturing of integrated circuits and devices. Overlay accuracy generally pertains to the determination of how accurately a first patterned layer aligns with respect to a second patterned layer disposed above or below it and to the determination of how accurately a first pattern aligns with respect to a second pattern disposed on the same layer. Presently, overlay measurements are performed via test patterns that are printed together with layers of the wafer. The images of these test patterns are captured via an imaging tool and an analysis algorithm is used to calculate (e.g., calculated using a computing system 124 coupled to an output of the detector 106) the relative displacement of the patterns from the captured images. Such overlay metrology targets (or ‘marks’) generally comprise features formed in two layers, the features configured to enable measurement of spatial displacement between features of the layers (i.e., the overlay or displacement between layers).
Overlay measurement precision, however, is limited by the level of achievable contrast in a given metrology system. Contrast in an optical metrology system is generally constrained by the peak to valley difference in the image projection of the lowest contrast target feature. Further, metrology accuracy and tool induced shift (TIS) performance is limited by contrast, also generally constrained by the peak to valley difference in the image projection of the lowest contrast target feature. In many metrology target architectures, a contrast reversal of an edge or periodic feature may occur when illuminated from different angles of incidence (i.e., illuminated from different locations in the illumination pupil). When a target is simultaneously illuminated from multiple angles of incidence the effect of contrast reversal may act to reduce or even eliminate the observed contrast entirely when light from multiple angles of incidence are integrated in the image plane.
Conventional optical metrology systems control contrast utilizing fixed apertures and the lateral movements of fixed apertures (e.g., using piezoelectric control). The conventional systems are limited, in part, because of their binary nature (i.e., ON or OFF). Although existing targets and target measurement systems are suitable for many implementation contexts, it is contemplated herein that many improvements may be made. The invention described herein discloses methods and apparatuses which overcome the disadvantages of the prior art.